A rare condition in which the tissues in the brain become inflamed. Encephalitis is usually caused by a viral infection, but it may also be caused by bacterial or parasitic infections, cancer, or certain drugs or toxins. Signs and symptoms include fever, headache, joint or muscle aches, poor appetite, fatigue, weakness, confusion, changes in behavior, seizures, and problems with vision, speech, or movement. Infants and young children may have nausea, vomiting, body stiffness, feeding problems, and irritability. Most patients have mild symptoms, but sometimes symptoms may be severe or life threatening.